In recent years, many sensors have been mounted to a vehicle to collect a lot of vehicle information (e.g., speed) in order to accurately control many functions of the vehicle. The sensors are connected to a control unit via a communication cable and exchange information between one another.
In a conventional communication system shown in FIG. 9, a control unit 112 acting as a master controller is connected to a positive terminal of a battery 107 via an ignition switch 106 of a vehicle. A negative terminal of the battery 107 is connected to a frame ground FG, i.e., the negative terminal of the battery 107 is grounded to a frame (i.e., chassis) of the vehicle. A sensor apparatus 203 acting as a slave controller is connected to the control unit 112 via a communication cable 111 consisting of first and second wires. A sensor 202 acting as a local device is connected to the sensor apparatus 203.
The sensor apparatus 203 includes a power supply circuit (PS) 203a, a determination circuit (DT) 203h, and a communication interface circuit (I/O) 203i. The communication cable 111 is connected to the power supply circuit 203a via a first input terminal BA of the sensor apparatus 203. Also, the communication cable 111 is connected to the communication interface circuit 203i via a second input terminal BB of the sensor apparatus 203. The first and second wires of the communication cable 111 are connected to the first and second input terminals BA, BB, respectively. An output of the power supply circuit 203a is connected to a positive terminal 202g of the sensor 202 via a first output terminal SA of the sensor apparatus 203. A negative terminal 202h of the sensor 202 is connected to a signal ground SG of the sensor apparatus 203 via a second output terminal SB of the sensor apparatus 203.
As shown in FIG. 10, the control unit 112 has two phases, one of which is a feeding phase and the other of which is a communication phase. In the feeding phase, the control unit 112 feeds a first DC voltage with respect to the frame ground FG to the sensor apparatus 203 via the communication cable 111. In the commutation phase, the first DC voltage on the communication cable 111 is changed so that the control unit 112 communicates with the sensor apparatus 203. Specifically, in the communication phase, voltages on the first and second wires of the communication cable 111 are pulsed and opposite in phase. Accordingly, voltages at the first and second input terminals BA, BB of the sensor apparatus 203 are pulsed and opposite in phase, as shown in FIG. 10.
The power supply circuit 203a of the sensor apparatus 203 generates a second DC voltage from the first DC voltage and feeds the second DC voltage to the sensor 202. As shown in FIG. 11, in the feeding phase, the second DC voltage is fed with respect to the frame ground FG. However, in the communication phase, the second DC voltage varies with the first DC voltage and consequently is fed with respect to a potential higher than the frame ground FG. Further, the second DC voltage is pulsed synchronously with the first DC voltage such that voltages on the first and second output terminals SA, SB of the sensor apparatus 203 are in phase with each other. Therefore, if wires connecting the sensor 202 and the sensor apparatus 203 are long or the sensor 202 is constructed of linear conductors, the wires or the sensor 202 itself may act as an antenna and emit noise.
A communication system disclosed in JP-A-2005-277546 is designed to prevent the emission of noise. The communication system includes a master controller, a slave controller, and a communication cable for connecting the master and slave controllers. The slave controller is provided with a termination circuit. The termination circuit matches impedances between the slave controller and the communication cable, regardless of transition of the potential on the communication cable. Thus, impedance mismatching is prevented so that noise emitted by the communication cable and the slave controller can be reduced.
However, in the communication system shown in FIG. 9, the noise is caused by the fact that the second DC voltage is pulsed synchronously with the first DC voltage such that the voltages on the first and second terminals SA, SB are in phase with each other. In short, the impedance mismatching does not cause the noise in the communication system shown in FIG. 9. Therefore, the termination circuit used in the communication system disclosed in JP-A-2005-277546 cannot reduce the noise in the communication system shown in FIG. 9.